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17.2.2.2 Replication Metadata Repositories

A replica server creates two replication metadata repositories, the connection metadata repository and the applier metadata repository. The replication metadata repositories survive a replica server's shutdown. If binary log file position based replication is in use, when the replica restarts, it reads the two repositories to determine how far it previously proceeded in reading the binary log from the source and in processing its own relay log.

  • The replica's connection metadata repository contains information that the replication I/O thread needs to connect to the replication source server and retrieve transactions from the source's binary log. The metadata in this repository includes the connection configuration, the replication user account details, the SSL settings for the connection, and the file name and position where the replication I/O thread is currently reading from the source's binary log.

  • The replica's applier metadata repository contains information that the replication SQL thread needs to read and apply transactions from the replica's relay log. The metadata in this repository includes the file name and position up to which the replication SQL thread has executed the transactions in the relay log, and the equivalent position in the source's binary log. It also includes metadata for the process of applying transactions, such as the number of worker threads.

By default, the replication metadata repositories are created as files in the data directory named master.info and relay-log.info, or with alternative names and locations specified by the --master-info-file option and relay_log_info_file system variable. To create the replication metadata repositories as tables, specify master_info_repository=TABLE and relay_log_info_repository=TABLE at server startup. In that case, the replica's connection metadata repository is written to the slave_master_info table in the mysql system schema, and the replica's applier metadata repository is written to the slave_relay_log_info table in the mysql system schema. A warning message is issued if mysqld is unable to initialize the tables for the replication metadata repositories, but the replica is allowed to continue starting. This situation is most likely to occur when upgrading from a version of MySQL that does not support the use of tables for the repositories to one in which they are supported.

Important
  1. Do not attempt to update or insert rows in the mysql.slave_master_info or mysql.slave_relay_log_info tables manually. Doing so can cause undefined behavior, and is not supported. Execution of any statement requiring a write lock on either or both of the slave_master_info and slave_relay_log_info tables is disallowed while replication is ongoing (although statements that perform only reads are permitted at any time).

  2. Access to the replica's connection metadata repository file or table should be restricted to the database administrator, because it contains the replication user account name and password for connecting to the source. Use a restricted access mode to protect database backups that include this repository.

RESET SLAVE clears the data in the replication metadata repositories, with the exception of the replication connection parameters (depending on the MySQL Server release and repository type). For details, see the description for RESET SLAVE.

In MySQL 5.6.5 and earlier, the slave_master_info and slave_relay_log_info tables used MyISAM by default, which meant that it was necessary before starting replication to change the storage engine used by these tables by issuing ALTER TABLE ... ENGINE=InnoDB, as shown here:

ALTER TABLE mysql.slave_master_info ENGINE=InnoDB;
ALTER TABLE mysql.slave_relay_log_info ENGINE=InnoDB;

The ALTER TABLE statements must be executed by the MySQL root or other user account with the appropriate privileges on the mysql system database. You should not attempt to do this while replication is running; beginning with MySQL 5.6.3, trying to execute an ALTER TABLE on either these tables while replication is ongoing is disallowed.

From MySQL 5.6.6, if you set master_info_repository and relay_log_info_repository to TABLE, the mysql.slave_master_info and mysql.slave_relay_log_info tables are created using the InnoDB transactional storage engine. As a table, updates to the replica's applier metadata repository are committed together with the transactions, meaning that the replica's progress information recorded in that log is always consistent with what has been applied to the database, even in the event of an unexpected server halt. The --relay-log-recovery option must be enabled on the replica to guarantee resilience. For more details, see Section 17.3.2, “Handling an Unexpected Halt of a Replica Server”.

When you back up the replica's data or transfer a snapshot of its data to create a new replica, ensure that you include the mysql.slave_master_info and mysql.slave_relay_log_info tables containing the replication metadata repositories, or the equivalent files (master.info and relay-log.info in the data directory, unless you specified alternative names and locations). When binary log file position based replication is in use, the replication metadata repositories are needed to resume replication after restarting the restored or copied replica. If you do not have the relay log files, but still have the replica's applier metadata repository, you can check it to determine how far the replication SQL thread has executed in the source's binary log. Then you can use a CHANGE MASTER TO statement with the MASTER_LOG_FILE and MASTER_LOG_POS options to tell the replica to re-read the binary logs from the source from that point (provided that the required binary logs still exist on the source).

One additional repository, the applier worker metadata repository, is created primarily for internal use, and holds status information about worker threads on a multithreaded replica. The applier worker metadata repository includes the names and positions for the relay log file and the source's binary log file for each worker thread. If the replica's applier metadata repository is created as a table, which is the default, the applier worker metadata repository is written to the mysql.slave_worker_info table. If the applier metadata repository is written to a file, the applier worker metadata repository is written to the worker-relay-log.info file. For external use, status information for worker threads is presented in the Performance Schema replication_applier_status_by_worker table.

The replication metadata repositories originally contained information similar to that shown in the output of the SHOW SLAVE STATUS statement, which is discussed in Section 13.4.2, “SQL Statements for Controlling Replica Servers”. Further information has since been added to the replication metadata repositories which is not displayed by the SHOW SLAVE STATUS statement.

For the connection metadata repository, the following table shows the correspondence between the columns in the mysql.slave_master_info table, the columns displayed by SHOW SLAVE STATUS, and the lines in the master.info file.

master.info File Line slave_master_info Table Column SHOW SLAVE STATUS Column Description
1 Number_of_lines [None] Number of lines in the file, or columns in the table
2 Master_log_name Master_Log_File The name of the binary log currently being read from the source
3 Master_log_pos Read_Master_Log_Pos The current position within the binary log that has been read from the source
4 Host Master_Host The host name of the source server
5 User_name Master_User The replication user name used to connect to the source
6 User_password Password (not shown by SHOW SLAVE STATUS) The password used to connect to the source
7 Port Master_Port The network port used to connect to the source
8 Connect_retry Connect_Retry The period (in seconds) that the replica will wait before trying to reconnect to the source
9 Enabled_ssl Master_SSL_Allowed Indicates whether the server supports SSL connections
10 Ssl_ca Master_SSL_CA_File The file used for the Certificate Authority (CA) certificate
11 Ssl_capath Master_SSL_CA_Path The path to the Certificate Authority (CA) certificates
12 Ssl_cert Master_SSL_Cert The name of the SSL certificate file
13 Ssl_cipher Master_SSL_Cipher The list of possible ciphers used in the handshake for the SSL connection
14 Ssl_key Master_SSL_Key The name of the SSL key file
15 Ssl_verify_server_cert Master_SSL_Verify_Server_Cert Whether to verify the server certificate
16 Heartbeat [None] Interval between replication heartbeats, in seconds
17 Bind Master_Bind Which of the replica's network interfaces should be used for connecting to the source
18 Ignored_server_ids Replicate_Ignore_Server_Ids The list of server IDs to be ignored. Note that for Ignored_server_ids the list of server IDs is preceded by the total number of server IDs to ignore.
19 Uuid Master_UUID The source's unique ID
20 Retry_count Master_Retry_Count Maximum number of reconnection attempts permitted
21 Ssl_crl [None] Path to an SSL certificate revocation-list file
22 Ssl_crl_path [None] Path to a directory containing SSL certificate revocation-list files
23 Enabled_auto_position Auto_position If autopositioning is in use or not (added in MySQL version 5.6.5)

For the applier metadata repository, the following table shows the correspondence between the columns in the mysql.slave_relay_log_info table, the columns displayed by SHOW SLAVE STATUS, and the lines in the relay-log.info file.

Line in relay-log.info slave_relay_log_info Table Column SHOW SLAVE STATUS Column Description
1 Number_of_lines [None] Number of lines in the file or columns in the table
2 Relay_log_name Relay_Log_File The name of the current relay log file
3 Relay_log_pos Relay_Log_Pos The current position within the relay log file; events up to this position have been executed on the replica database
4 Master_log_name Relay_Master_Log_File The name of the source's binary log file from which the events in the relay log file were read
5 Master_log_pos Exec_Master_Log_Pos The equivalent position within the source's binary log file of events that have already been executed
5 Sql_delay SQL_Delay The number of seconds that the replica must lag the source
6 Number_of_workers [None] The number of worker threads on the replica for executing replication events (transactions) in parallel
7 Id [None] ID used for internal purposes; currently this is always 1

Prior to MySQL 5.6, the relay-log.info file does not include a line count or a delay value (and the slave_relay_log_info table is not available).

Line Status Column Description
1 Relay_Log_File The name of the current relay log file
2 Relay_Log_Pos The current position within the relay log file; events up to this position have been executed on the replica database
3 Relay_Master_Log_File The name of the source's binary log file from which the events in the relay log file were read
4 Exec_Master_Log_Pos The equivalent position within the source's binary log file of events that have already been executed
Note

If you downgrade a replica server to a version older than MySQL 5.6, the older server does not read the relay-log.info file correctly. To address this, modify the file in a text editor by deleting the initial line containing the number of lines.

The contents of the relay-log.info file and the states shown by the SHOW SLAVE STATUS statement might not match if the relay-log.info file has not been flushed to disk. Ideally, you should only view relay-log.info on a replica that is offline (that is, mysqld is not running). For a running system, you can use SHOW SLAVE STATUS, or query the slave_master_info and slave_relay_log_info tables if you are writing the replication metadata repositories to tables.